Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
In ink jet recording processes, it is necessary that the inks being used meet various performance requirements. Such performance requirements are generally more stringent than those for other liquid ink applications, such as for writing instruments (e.g., a fountain pen, felt pen, etc.). In particular, the following conditions are generally required for inks utilized in ink jet printing processes:    (1) The ink should possess physical properties such as viscosity, surface tension, and electric conductivity matching the discharging conditions of the printing apparatus, such as the driving voltage and driving frequency of a piezoelectric electric oscillator, the form and material of printhead orifices, the diameter of orifices, etc;    (2) The ink should be capable of being stored for a long period of time without causing clogging of printhead orifices during use;    (3) The ink should be quickly fixable onto recording media, such as paper, film, etc., such that the outlines of the resulting ink dots are smooth and there is minimal blotting of the dotted ink;    (4) The printed image should be of high quality, such as having a clear color tone and high density, have high gloss and high color gamut;    (5) The printed image should exhibit excellent waterfastness (water resistance) and lightfastness (light resistance);    (6) The printed (ink) images should have good adhesion to the surface of image receiving elements and should be durable and highly resistant to physical and mechanical scratches or damages    (7) The ink should not chemically attack, corrode or erode surrounding materials such as the ink storage container, printhead components, orifices, etc;    (8) The ink should not have an unpleasant odor and should not be toxic or inflammable; and    (9) The ink should exhibit low foaming and high pH stability characteristics.
The inks used in various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic cosolvents. Each individual dye molecule is surrounded by molecules of the carrier medium. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor lightfastness. When water is used as the carrier medium, such inks also generally suffer from poor waterfastness.
Pigment-based inks have been gaining in popularity as a means of addressing these limitations. In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and/or settling out.
Pigment-based inks suffer from a different set of deficiencies than dye-based inks. One deficiency is that pigment-based inks interact differently with specially coated papers and films, such as transparent films used for overhead projection and glossy papers and opaque white films used for high quality graphics and pictorial output. In particular, it has been observed that pigment-based inks produce imaged areas that are entirely on the surface of coated papers and films which results in images that have poor dry and wet adhesion properties and can be easily smudged. In recent years, ink jet receivers have been developed to have both high gloss and fast drying capacities. However, scratch mark smudges are more visible on high gloss receivers. There is a need to provide a pigmented ink composition that results in images on the surface of an ink jet recording element which have improved durability and smudging resistance.
Another deficiency from which pigmented inks suffer is their poor storage stability due to the presence of a water-miscible organic solvent. Water-miscible organic solvents are used to adjust ink rheology, to maximize ink firability and re-runability. These solvents prevent ink from drying in a printing head and lower ink surface tension to minimize the effect of air-entrapment in an ink formulation which would otherwise generate air bubbles which would seriously affect the head performance. However, these water-miscible organic solvents can also have a negative effect on the colloidal stability of pigment particles in an ink formulation. There is a need to provide a pigmented ink composition which would have improved storage stability in the presence of various types of water-miscible organic solvents.
U.S. Pat. No. 5,852,073 discloses an erasable ink composition which comprises a dispersion of particles of water-insoluble polymer-encapsulated colorant obtained by polymerizing under emulsion polymerization conditions in the presence of solid colorant particles. The process described is a batch or semi-batch process by which an ethylenically-unsaturated monomer and a colorant are emulsified together before being subjected to emulsion polymerization conditions. The particle dispersions prepared by such a process are a physical mixture of polymer latex particles, polymer encapsulated colorant, and water soluble polymers. However, there is a problem with these dispersions when they are formulated into an ink jet ink, in that the presence of free polymers (both polymer particles and water soluble polymers) can significantly increase ink viscosity, decrease ink storage stability, cause premature printing head failure, and generate image defects.
EP1006161 discloses a process for making an ink composition wherein polymer-coated colorant particles are prepared using a batch emulsion polymerization process employing colorant particles, monomers and an initiator. The polymer-coated colorant particles prepared by such a process are a physical mixture of polymer particles, polymer-encapsulated colorant and water-soluble polymers. However, there is a problem with these dispersions when they are formulated into an ink jet ink, in that the presence of free polymers (both polymer particles and water soluble polymers) can significantly increase ink viscosity, decrease ink storage stability, cause premature printing head failure, and generate image defects.
It is an object of this invention to provide an ink jet ink composition containing composite colorant polymer particles that have improved compatibility with water-soluble organic solvents, improved stability in an ink jet composition, and when such ink composition is printed, the resultant images on the surface of an ink jet recording element have improved image quality and physical durability such as scratch and smudging resistance.